Everytime I reboot my HP dm4 Pavillion the brightness is zero on my screen. BTW I use a Ubuntu 12.04..
You can try this:-
edit the file /etc/rc.local
before the last line 'exit 0' add the following line:
echo 4 > /sys/class/backlight/acpi_video0/brightness
save and reboot.
This should then set your brightness. I will add the caveat that this doesn't always work with HP machines but hopefully you'll be OK.